How to improve Internal combustion Engine

5 Ways to Redesign the Internal Combustion Engine

Since the earliest days of the automobile in late 19th century, the dominant powerplant has been the reciprocating piston, spark-ignition Otto-cycle, followed by the compression-ignition diesel. Those engines are still undergoing plenty of development efforts and will see significant improvements in fuel efficiency in the next few years, thanks to direct injection, turbocharging and, further out, homogeneous charge compression ignition (HCCI). However, all these engines feature the same piston-connecting rod-crankshaft architecture. As engineers strive for ever more efficiency, new architectures are being investigated and older less-successful types are being revisited.                                             

Stirling 01.

Each of the alternative engine architectures discussed here have one major feature in common with the standard piston engines that have dominated the automobile for more than a century: Fuel is burned inside a chamber to convert chemical energy into mechanical energy for propulsion. However, that requires moving air and fuel in and exhaust gases out of the combustion chamber, all of which adds complexity and reduces efficiency.


In 1816, Scottish inventor Robert Stirling conceived of the closed-cycle engine with the working fluid (in this case, air) remaining contained within the device. The heat source—which could be almost anything, including combustion—is external to the engine. Like the Ecomotors OPOC and the Scuderi, pairs of pistons operate together to provide the complete cycle. The air in one chamber is heated via heat transfer through the cylinder wall pushing back the displacer piston, which is linked to a second power piston in the expansion chamber. As the heated air continues to expand, it displaces the power piston, which drives a crankshaft that produces rotational torque. As the air cools, both pistons move back to their original positions, and the process repeats.

Until recently, Stirling engines were mainly used for stationary applications—in part because they were not suitable for typical transient applications where the power delivery varied significantly over time. However, newer configurations and the ability to use alternative fuels have revived interest, especially for range-extender applications where constant speed operation and low noise (due to the continuous external combustion) are beneficial. 

alt_engines_02

 

The opposed-piston opposed-cylinder (OPOC) architecture has drawn considerable attention recently with the emergence of a new company called Ecomotors. Ecomotors includes numerous veteran auto-industry executives and engineers, including Don Runkle of General Motors and Peter Hofbauer, formerly of Volkswagen.

The primary claimed advantage of the OPOC architecture is high power density and fuel efficiency improvements of 50 percent over current spark-ignition engines. Ecomotors has developed a modular configuration with each module consisting of two cylinders. Within each cylinder are two pistons that are linked to a common crankshaft. The pairs of pistons oscillate back and forth with a common combustion chamber between them. The OPOC engine operates on a two-stroke cycle, with each piston exposing only the intake or exhaust ports, allowing better management of which ports are open by timing each piston.

Hofbauer explains that the use of two pistons per cylinder allows the pistons to move only half the distance for the same compression ratio so that the engine can run twice as fast. Like many of these alternative architectures, the OPOC engine can run on a variety of fuels including both gasoline and diesel as well as biofuels. Modules of two cylinders each can be joined together providing as much power as needed for a given application while electronically controlled clutches allow the individual modules to be shut down for reduced fuel consumption during light loads. 

Scuderi

For more than a century, virtually all the engines we've used have operated on either a two- or four-stroke Diesel or Otto cycle, with the entire combustion cycle taking place within any number of single cylinders. Each cylinder would have intake, compression, power and exhaust activities. The idea of the split cycle—in which one cylinder handles intake and compression and a second handles power and exhaust—dates back to at least the late 19th century, yet no one has ever had much success with it.

The Scuderi Group hopes to change that with a split-cycle design it has been developing over the last several years. Each engine module consists of two cylinders and pistons tied together through the crankshaft and a high-pressure crossover passage. Because only air is being squeezed into the first cylinder, it has 75:1 compression ratio. The outlet valve of cylinder one releases the high-pressure air into a crossover passage where some cooling occurs.

When the inlet to the second cylinder opens as that piston approaches the top of its stroke, the high-pressure air rushes in from the crossover. After the valve closes, fuel is injected and ignited about 15 degrees past top dead center. This timing ensures that the air is not recompressed, which improves overall thermodynamic efficiency. Scuderi claims a normally aspirated version of its engine can produce up to 135 hp per liter, giving it much better power density and lower fuel consumption than conventional engines. An air-hybrid version using a high-pressure accumulator that is charged during vehicle coast-down could improve efficiency by another 50 percent. The Scuderi concept is compatible with spark-ignition operation on gasoline and other fuels or compression ignition with diesel fuel. The first functional Scuderi engine began testing on a dynamometer in mid-2009, and the company hopes to strike a production deal with an automaker within five years. 

Free-Piston

The free-piston engine has some similarities to the OPOC but generally only uses two pistons per module. The pistons are attached to each end of a solid connecting rod and oscillate back and forth in the cylinder, alternately firing each piston on a two-stroke cycle. Free-piston engines have lower friction than traditional crankshaft-based piston engines as a result of reduced rotary motion. A free-piston engine can achieve up to 50 percent thermodynamic efficiency, or about double the efficiency of a conventional gasoline engine. However, that same lack of rotary motion makes this design problematic for use as a propulsion unit.

One architectural configuration of the free-piston engine that could prove useful in the future is to use it as a generator for an extended range electric vehicle. Copper windings around the central section of the cylinder could be combined with magnets on the connecting rod to generate electricity that would be used to charge a battery. The compact size of the engine and nearly vibration-free operation make this a viable alternative for these electrically driven cars.

Wankel

Felix Wankel's rotary design is not exactly a new engine architecture, having been used in a variety of production cars since he completed the first running prototype in 1957. Like several of the other architectures discussed here, the Wankel has the benefit of very high power density. The current 1.3-liter normally aspirated two-rotor design used by Mazda in the RX-8 sports car generates 238 hp. Unfortunately, Wankels have had issues with high fuel and oil consumption, which has limited their use in recent decades.

However, several modern developments have made a revival of the Wankel a distinct possibility. New machining processes can provide much-improved surface finish on the chamber walls, and new seal materials can reduce oil consumption and improve durability. The addition of direct fuel injection will facilitate reduced fuel consumption and emissions by preventing unburned fuel from flowing out through the ports as the rotor sweeps by.

The emergence of extended range electric vehicles (ER-EV), like the Chevrolet Volt, has suddenly provided a seemingly ideal application for Wankels. Because the engine in these vehicles is only used to drive a generator, it can be optimized for operation at certain fixed speeds rather than transient operation. The compact dimensions also make it easier to package in this type of vehicle, and its vibration-free operation allows more seamless charge-sustaining operation. At the 2010 Geneva Motor Show, Audi showed an ER-EV concept based on its new sub-compact A1 that uses a Wankel range extender, and powertrain engineering consultants AVL and FEV have both shown similar demonstration vehicles in recent months. Even General Motors has acknowledged investigating the use of a Wankel for future generations of the Volt.

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